Electrochemical reaction device

ABSTRACT

An electrochemical reaction device includes: an electrolytic solution tank including a first storage part to store a first electrolytic solution, and a second storage part to store a second electrolytic solution; a reduction electrode disposed in the first storage part and having a first surface; an oxidation electrode disposed in the second storage part and having a second surface; and a generator connected to the reduction and oxidation electrodes. A region in the first storage part between the first surface and an inner wall of the first storage part is an electrolytic solution flow path. The electrolytic solution flow path has a maximum part and a minimum part.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority from Japanese Patent Applications No. 2016-037675, filed on Feb. 29, 2016; the entire contents of all of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to an electrochemical reaction device.

BACKGROUND

Artificial photosynthesis is a system modeling photosynthesis of a living world in a wide meaning and is a technology of reducing carbon dioxide using sunlight into resource.

In the highly developed contemporary society, economic activities are performed using fossil fuel as a main energy source. However, the consumption of fossil fuel increases due to an increase in population and activization of economic activities in developing countries, and depletion of fossil fuel is worried as a real and common problem. In the contemporary society, utilization of limited resources and energy promotes the development of the society and is responsibility of modems. The artificial photosynthesis technology is a technology of recycling carbon dioxide that is a contributor to global warming and the final form of the fossil fuel after use, and converting carbon dioxide into fuel by using inexhaustible solar energy as an energy source. Accordingly, the technology can be said to be an ultimate technology of generating green energy.

In recent years, renewable energy is attracting attention. Of this, the solar energy is attractive as an unlimited energy source. An example of a solar power generation system is a large-scale solar power generation system called mega solar. The product by the solar power generation system is electric energy, and therefore a transmission system and a power storage system need to be additionally provided. In contrast to this, recycling carbon dioxide by the artificial photosynthesis directly generates fuel gas, liquid or the like such as hydrogen, methane, carbon monoxide (Fischer-Tropsch synthesis (FT method) or methanol synthesis reaction with hydrogen is required for converting carbon monoxide into fuel), methanol, or ethylene glycol which can be converted from sunlight into fuel from sunlight. For this reason, as long as a conveying facility is provided, no additional facility is required. An artificial photosynthesis system is suitable for a hydrogen society which is expected to be put into practical use in the future, and has possibility as a plant of producing fuel standing alone even in a desert.

As an electrochemical reaction device that electrochemically converts sunlight to a chemical substance, there has been known, for example, a two-electrode type device that includes an electrode having a reduction catalyst for reducing carbon dioxide (CO₂) and an electrode having an oxidation catalyst for oxidizing water (H₂O), and in which these electrodes are immersed in water with CO₂ dissolved therein. These electrodes are electrically connected to each other via an electric wire or the like. In the electrode having the oxidation catalyst, H₂O is oxidized by light energy, whereby oxygen (½O₂) is obtained and a potential is obtained. In the electrode having the reduction catalyst, by obtaining the potential from the electrode in which the oxidation reaction is caused, carbon dioxide is reduced and formic acid (HCOOH) or the like is produced. As described above, in the two-electrode type device, the reduction potential of carbon dioxide is obtained by two-stage excitation, and therefore the conversion efficiency from the sunlight to chemical energy is low.

An electrochemical reaction device using a stack (silicon solar battery or the like) in which a photoelectric conversion body is sandwiched between a pair of electrodes, is also under investigation. In the electrode on a light irradiated side, water (2H₂O) is oxidized by light energy, whereby oxygen (O₂) and hydrogen ions (4H) are obtained. In the opposite electrode, by using the hydrogen ions (4H) generated in the electrode on the light irradiated side and a potential (e) generated in the photoelectric conversion body, hydrogen (2H₂) or the like is obtained as a chemical substance. Besides, an electrochemical reaction device made by stacking silicon solar batteries is also known. The electrochemical reaction device preferably has a high conversion efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view illustrating a structure example of an electrochemical reaction device.

FIG. 2 is a schematic exterior view illustrating the structure example of the electrochemical reaction device.

FIG. 3 is a chart illustrating the relationship between the reaction time and the current density.

FIG. 4 is a schematic view for explaining the thickness of a diffusion layer in a real space.

FIG. 5 is a chart illustrating the relationship between the reaction time and the thickness of a diffusion layer.

FIG. 6 is a chart illustrating the relationship between the radius of a cylindrical tube and the dimensionless velocity;

FIG. 7 is a schematic view illustrating a structure example of a photoelectric conversion cell.

FIG. 8 is a schematic sectional view illustrating another structure example of the electrochemical reaction device.

FIG. 9 is a schematic sectional view illustrating another structure example of the electrochemical reaction device.

FIG. 10 is a schematic sectional view illustrating another structure example of the electrochemical reaction device.

FIG. 11 is a schematic sectional view illustrating another structure example of the electrochemical reaction device.

FIG. 12 is a schematic sectional view illustrating another structure example of the electrochemical reaction device.

FIG. 13 is a schematic sectional view illustrating another structure example of the electrochemical reaction device.

FIG. 14 is a schematic sectional view illustrating another structure example of the electrochemical reaction device.

DETAILED DESCRIPTION

An electrochemical reaction device in an embodiment includes: an electrolytic solution tank including a first storage part to store a first electrolytic solution containing carbon dioxide, and a second storage part to store a second electrolytic solution containing water; a reduction electrode disposed in the first storage part and having a first surface containing a reduction catalyst; an oxidation electrode disposed in the second storage part and having a second surface containing an oxidation catalyst; and a generator connected to the reduction electrode and the oxidation electrode. A region in the first storage part between the first surface and an inner wall of the first storage part is an electrolytic solution flow path for sending the first electrolytic solution. The electrolytic solution flow path has a maximum part having a maximum value of an area distribution from one end to another end on a length direction of the electrolytic solution flow path of a cross section perpendicular to the length direction, and a minimum part having a minimum value of the area distribution.

Hereinafter, embodiments will be described with reference to the drawings. Note that the drawings are schematic and, for example, dimensions such as thickness and width of components may differ from actual dimensions of the components. Besides, in the embodiments, substantially the same components are denoted by the same reference signs and the description thereof will be omitted in some cases. A term of “connect” in the specification is not limited to a case of connecting directly but may include a meaning of connecting indirectly.

FIG. 1 is a schematic sectional view illustrating a structure example of an electrochemical reaction device. FIG. 2 is a schematic view of an external appearance of the electrochemical reaction device. The electrochemical reaction device includes, as illustrated in FIG. 1 and FIG. 2, an electrolytic solution tank 11, a reduction electrode 31, an oxidation electrode 32, a photoelectric conversion body 33, and an ion exchange membrane 4.

The electrolytic solution tank 11 has a storage part 111 and a storage part 112. The shape of the electrolytic solution tank 11 is not particularly limited as long as it is a solid shape having space portions being the storage parts. Examples of usable material of the electrolytic solution tank 11 include glass such as pyrex glass, blue plate glass, and quartz glass, and organic matters such as polyimide, polyacryl, polymethylmethacrylate, polyethylene terephthalate and the like. Since the strength and durability against flow are required, a polyacrylic resin is preferable. The storage part 111 and storage part 112 are required to have a light transmission property on their light incident surface sides. Flow paths on the opposite side to the light incident surfaces are not required to have light a transmission property, and an ordinary flow path material may be used. For example, any of polyvinyl chloride, polyimide subjected to Teflon treatment, polyacryl, ordinary glass, ceramics, concrete material and the like can be arbitrarily selected. In this case, preferably, an optimal material is selected from the viewpoint of weight and total cost.

The storage part 111 stores an electrolytic solution 21 containing a substance to be reduced. The substance to be reduced is a substance that is reduced by a reduction reaction. The substance to be reduced contains carbon dioxide. The substance to be reduced may further contain hydrogen ions. Changing the amount of water and electrolytic solution components contained in the electrolytic solution 21 can change the reactivity and thereby change the selectivity of the substance to be reduced and the ratio of a chemical substance to be produced.

The storage part 112 stores an electrolytic solution 22 containing a substance to be oxidized. The substance to be oxidized is a substance that is oxidized by an oxidation reaction. The substance to be oxidized contains, for example, water. The substance to be oxidized may contain an organic matter such as alcohol or amine and an inorganic oxide such as an iron oxide. The electrolytic solution 22 may contain the same substance as that in the electrolytic solution 21. In this case, the electrolytic solution 21 and the electrolytic solution 22 may be recognized as one electrolytic solution.

The pH of the electrolytic solution 22 is preferably higher than the pH of the electrolytic solution 21. This facilitates migration of hydrogen ions, hydroxide ions and the like. A liquid junction potential due to the different in pH allows oxidation-reduction reaction to effectively proceed.

The reduction electrode 31 has a surface 311 containing a reduction catalyst. The reduction electrode 31 is immersed in the electrolytic solution 21. In this event, the surface 311 comes into contact with the electrolytic solution 21. A compound to be produced by the reduction reaction differs depending on the kind of the reduction catalyst or the like. The compound to be produced by the reduction reaction is, for example, a carbon compound such as carbon monoxide (CO), formic acid (HCOOH), methane (CH₄), methanol (CH₃OH), ethane (C₂H₆), ethylene (C₂H₄), ethanol (C₂H₅OH), formaldehyde (HCHO), or ethylene glycol (HOCH₂CH₂OH); or hydrogen. The compound produced by the reduction reaction may be recovered through, for example, a product flow path. In this event, the product flow path is connected, for example, to the storage part 111. The compound produced by the reduction reaction may be recovered through another flow path.

The reduction electrode 31 may have a structure of, for example, a thin-film shape, a lattice shape, a granular shape, or a wire shape. The reduction electrode 31 does not necessarily have to be provided with the reduction catalyst. A reduction catalyst layer provided outside the reduction electrode 31 may be electrically connected to the reduction electrode 31.

The oxidation electrode 32 has a surface 321 containing an oxidation catalyst. The oxidation electrode 32 is immersed in the electrolytic solution 22. In this event, the surface 321 comes into contact with the electrolytic solution 22. A compound to be produced by the oxidation reaction differs depending on the kind of the oxidation catalyst or the like. The compound to be produced by the oxidation reaction is, for example, hydrogen ions. The compound produced by the oxidation reaction may be recovered through, for example, a product flow path. In this event, the product flow path is connected, for example, to the storage part 112. The compound produced by the oxidation reaction may be recovered through another flow path.

The oxidation electrode 32 may have a structure of, for example, a thin-film shape, a lattice shape, a granular shape, or a wire shape. The oxidation electrode 32 does not necessarily have to be provided with the oxidation catalyst. An oxidation catalyst layer provided other than the oxidation electrode 32 may be electrically connected to the oxidation electrode 32.

In the case where the oxidation electrode 32 is stacked and immersed in the electrolytic solution 22 and the photoelectric conversion body 33 is irradiated with light via the oxidation electrode 32 to perform the oxidation-reduction reaction, the oxidation electrode 32 needs to have a light transmitting property. The light transmittance of the oxidation electrode 32 is preferably, for example, at least 10% or more of an irradiation amount of the irradiating light to the oxidation electrode 32, more preferably 30% or more thereof, and furthermore preferably 80% or more thereof. Not limited to this, but the photoelectric conversion body 33 may be irradiated with light, for example, via the reduction electrode 31.

The photoelectric conversion body 33 has a surface 331 electrically connected to the reduction electrode 31 and a surface 332 electrically connected to the oxidation electrode 32. In FIG. 1, the surface 331 is in contact with a surface opposite to the surface 311, and the surface 332 is in contact with a surface opposite to the surface 321. Note that connection between the surface 331 and the surface opposite to the surface 311, and connection between the surface 332 and the surface opposite to the surface 321 may be made by heat transfer members such as wires having a heat transfer property. The case of connecting the photoelectric conversion body and the reduction electrode or the oxidation electrode by the wire or the like, is advantageous in terms of a system because the components are separated for each function. The photoelectric conversion body 33 may be provided outside the electrolytic solution tank 11. Note that the photoelectric conversion body 33 does not necessarily have to be provided. Another generator may be connected to the oxidation electrode 32 and the reduction electrode 31. Examples of the generator include a system power supply, a storage battery, or the renewable energy such as the wind power, water power, and the geothermal power. A cell including the reduction electrode 31, the oxidation electrode 32, and the photoelectric conversion body 33 is also referred to as a photoelectric conversion cell.

The photoelectric conversion body 33 has a function of performing charge separation by energy of irradiating light such as sunlight. Electrons generated by the charge separation move to the reduction electrode side and holes move to the oxidation electrode side. This allows the photoelectric conversion body 33 to generate electromotive force. As the photoelectric conversion body 33, a photoelectric conversion body of a pn junction type or a pin junction type can be used. The photoelectric conversion body 33 may be fixed, for example, to the electrolytic solution tank 11. Note that the photoelectric conversion body 33 may be formed by stacking a plurality of photoelectric conversion bodies. The sizes of the reduction electrode 31, the oxidation electrode 32, and the photoelectric conversion body 33 may be different from one another.

The ion exchange membrane 4 is provided in a manner to separate the storage part 111 and the storage part 112. As the ion exchange membrane 4, for example, Neosepta (registered trademark) of Astom Corporation, Selemion (registered trademark) or Aciplex (registered trademark) of Asahi Glass Corporation, Ltd., Fumasep (registered trademark) or fumapem (registered trademark) of Fumatech Corporation, Nafion (registered trademark) of DuPont Corporation being a fluorocarbon resin made by performing sulfonation and polymerization on tetrafluoroethylene, lewabrane (registered trademark) of LANXESS Corporation, IONSEP (registered trademark) of IONTECH Corporation, Mustang (registered trademark) of PALL Corporation, ralex (registered trademark) of mega Corporation, Gore-Tex (registered trademark) of Gore-Tex Corporation, or the like can be used. Besides, the ion exchange membrane may be constituted of a membrane whose basic structure is hydrocarbon, or a membrane having an amine group in anion exchange. Note that the ion exchange membrane 4 does not necessarily have to be provided.

A region in the storage part 111 between the surface 311 and the inner wall of the storage part 111 is an electrolytic solution flow path 51 for sending at least a part of the electrolytic solution 21. The ions and other substances contained in the electrolytic solution 21 can move through the electrolytic solution flow path 51. A region in the storage part 112 between the surface 321 and the inner wall of the storage part 112 is an electrolytic solution flow path 52 for sending at least a part of the electrolytic solution 22. The ions and other substances contained in the electrolytic solution 22 can move through the electrolytic solution flow path 52. Outline arrows illustrated in FIG. 1 and FIG. 2 indicate the direction in which the electrolytic solutions can move. The electrolytic solution in at least one of the electrolytic solution flow path 51 and the electrolytic solution flow path 52 may be sent by a pump. As the pump, a tube pump, a plunger pump, a pressure pump, a piston pump, a magnet pump, a screw pump or the like can be used.

The electrolytic solution flow path 51 has a maximum part 51 a and a minimum part 51 b. Expressing an area distribution from one end to the other end of the electrolytic solution flow path 51 (an opposite portion to the surface 311) on a length direction of the electrolytic solution flow path 51 of the area of a cross-section perpendicular to the length direction, a largest cross-sectional area A of the maximum part 51 a is larger than a largest cross-sectional area B of the minimum part 51 b as illustrated in FIG. 1. More specifically, the maximum part 51 a has a maximum value of the area distribution and the minimum part 51 b has a minimum value of the area distribution. The maximum part 51 a and the minimum part 51 b may be provided alternately along, for example, the direction of the length of the electrolytic solution flow path 51.

The electrolytic solution flow path 51 has a partition wall 6 provided to be overlaid with the minimum part 51 b. In FIG. 1, the partition wall 6 is provided between the inner wall of the storage part 111 and the surface 311 of the reduction electrode 31, and is in contact with the inner wall of the storage part 111. As the material of the partition wall 6, for example, ordinary glass, polymer such as polyacryl, polyimide, vinyl chloride or the like can be used. Note that the partition wall 6 may be realized by forming a protruding portion at a part of the inner wall of the storage part 111.

Next, an operation example of the electrochemical reaction device illustrated in FIG. 1 and FIG. 2 will be described. When light enters the photoelectric conversion body 33, the photoelectric conversion body 33 generates photoexcited electrons and holes. In this event, the photoexcited electrons gather at the reduction electrode 31 and the holes gather at the oxidation electrode 32. This causes electromotive force in the photoelectric conversion body 33. The light is preferably sunlight, but light of a light-emitting diode or an organic EL may be made to enter the photoelectric conversion body 33.

A case of using electrolytic solutions containing water and carbon dioxide as the electrolytic solution 21 and the electrolytic solution 22 to produce carbon monoxide will be described. Around the catalyst layer 32, as expressed by the following formula (1), the oxidation reaction of water occurs to lose electrons and produce oxygen and hydrogen ions. At least one of the produced hydrogen ions migrates to the storage part 111 through the ion exchange membrane 4.

2H₂O→4H^(|)+O₂+4e ⁻  (1)

Around the reduction electrode 31, as expressed by the following formula (2), the reduction reaction of carbon dioxide occurs in which hydrogen ions react with carbon dioxide while receiving electrons to produce carbon monoxide and water. Carbon monoxide dissolves in the electrolytic solution 21 at an arbitrary ratio. Further, separately from the carbon monoxide, hydrogen ions receive electrons to produce hydrogen as expressed by the following formula (3). At this time, the hydrogen may be produced simultaneously with the carbon monoxide.

CO₂+2H⁺+2e ⁻→CO+H₂O  (2)

2H⁺+2e ⁻→H₂  (3)

The energy necessary for conversion from carbon dioxide to carbon monoxide is estimated to be 1.33 V from the difference in standard chemical energy. However, in the actual reduction of carbon dioxide, a voltage called an overvoltage necessary for the reaction, namely, a voltage larger than the theory is necessary. The overvoltage is used also for a reaction of oxidizing water to obtain oxygen, and the voltage at this time may reach, for example, several hundreds of millivolts to a little less than 2 V. Though depending on the designed current-voltage curve of the photoelectric conversion body, a value obtained by adding the overvoltage on the reduction side and the overvoltage on the oxidation side, a voltage added with a loss due to a solution resistance, and 1.33 V, is the voltage necessary for operation at a point intersecting the current-voltage curve.

The electrochemical reaction device in this embodiment has a maximum part and a minimum part which are provided only in the electrolytic solution flow path on the reduction electrode side and are different in area of the cross-section perpendicular to the length direction as described above. The flow of the electrolytic solution at the minimum part is a turbulent flow. The flow of the electrolytic solution at the maximum part is, for example, a laminar flow but not limited to this.

The difference between the laminar flow and the turbulent flow and a diffusion rate-controlling derived from a Fick's diffusion equation and the solubility and diffusion coefficient of carbon dioxide will be described. The carbon dioxide contained in the electrolytic solution on the reduction side is reduced in the electrolytic solution. However, the solubility of carbon dioxide in water is known to be 0.034 mol/l at 25° C. The above solubility is an extremely low solubility as a main reactant reacting in a module or a plant scale and means a low concentration of reactant.

Examples of the moving method of a substance in a solution include convection, diffusion, and ion drift. Since carbon dioxide is a neutral molecule, there is virtually no ion drift effect. Accordingly, the diffusion of carbon dioxide is mainly governed by Fick's first law and Fick's second law. The physical constants used in Fick's first law and Fick's second law are concentration and diffusion constant. The diffusion constant is physically decided, and in consideration of the concentration of 0.034 mol/l being the solubility limit amount, the following diffusion equation can be used to find the diffusion amount and the concentration gradient of carbon dioxide in the solution regarding Fick's first law (Mathematical expression (1)) and Fick's second law (Mathematical expression (2)).

$\begin{matrix} \left\lbrack {{Math}\mspace{14mu} 1} \right\rbrack & \; \\ {J = {D\left( \frac{dC}{dx} \right)}} & {{Mathematical}\mspace{14mu} {expression}\mspace{14mu} (1)} \end{matrix}$

C represents concentration, D represent diffusion constant, x represents position, J represent mass transfer, and t represents time. Assuming that the substance moves only in a one-dimensional direction, the flow flowing in a surface direction of the electrode is regarded to be positive. It is assumed that carbon dioxide reached the surface of the electrode immediately reacts. From the above, the solution of the one-dimensional diffusion equation is considered to satisfy the following relational expression (Mathematical expression (3)).

$\begin{matrix} {\left\lbrack {{Math}\mspace{14mu} 3} \right\rbrack \;} & \; \\ {j = {{nFJ} = \frac{{nFCme} \times \sqrt{D}}{\sqrt{\pi \; t}}}} & {{Mathematical}\mspace{14mu} {expression}\mspace{14mu} (3)} \end{matrix}$

J represents kinetic current density (mA/cm²), n represents reaction electron number, F represents Faraday constant (96500 c/mol), Cmax (mol/l) represents solubility limit amount of carbon dioxide, D represents diffusion constant of carbon dioxide (cm²/sec), and t represents reaction time (sec). The diffusion constant of carbon dioxide in an aqueous solution is known to be, for example, 1.92×10⁻⁵ cm²/sec. As for the reaction electron number n on the surface of the electrode, assuming the case where carbon dioxide is reduced to carbon monoxide, a two-electron reaction occurs and the current density is supposed to be positive.

FIG. 3 is a chart illustrating the relationship between the elapsed time and the current density obtained by calculating and plotting all parameters. Note that the natural convection by the molecular motion is ignored. The current density that is 16 mA/cm² after a reaction time of 1 second rapidly lowers, then becomes gentle in change to be almost constant in about 10 seconds. From the scientific knowledge, enough carbon dioxide exists on the surface of the electrode in a reaction time of 1 to 2 seconds, and the surface reaction can be said to be rate-controlling. Further after a lapse of several seconds, the curve rapidly lowers and then becomes gentle into a constant value. This can be understood as a process that the surface reaction rate-controlling state gradually changes to a diffusion rate-controlling state because carbon dioxide on the surface thoroughly reacts accompanying proceeding of the reaction. This means that carbon dioxide being the reactant changes to diffusion rate-controlling, and after a lapse of a certain time, comes to flow only by 1 to 2 mA/cm². This is a huge problem in the electrochemical reaction device. To satisfy a conversion efficiency of 10% of solar energy, the above-described problem in diffusion rate-controlling needs to be solved.

Next, the thickness of the diffusion layer in a real space will be described referring to the schematic view in FIG. 4. From the definition of the diffusion constant, the mass transfer speed J establishes the following relationship (Mathematical expression (4)).

$\begin{matrix} \left\lbrack {{Math}\mspace{14mu} 4} \right\rbrack & \; \\ {J = {D\left( \frac{dC}{dx} \right)}} & {{Mathematical}\mspace{14mu} {expression}\mspace{14mu} (4)} \end{matrix}$

To find a curve C(x) of the concentration gradient, the diffusion layer thickness is found using linear approximation. The above differential dc/dx is assumed to be macroscopic differential. More specifically, as the limit dx of Δx→0, the distance from x=0 to X where the concentration gradient starts is set as a diffusion layer thickness 6, and the saturated solubility of carbon dioxide is set as Cmax. Next, provided that carbon dioxide immediately reacts on the electrode surface and there is no residual carbon dioxide on the surface of the electrode, Mathematical expression (4) can be expressed as follows (Mathematical expression (5)).

$\begin{matrix} \left\lbrack {{Math}\mspace{14mu} 5} \right\rbrack & \; \\ {J = {D\left( \frac{C\; \max}{\delta} \right)}} & {{Mathematical}\mspace{14mu} {expression}\mspace{14mu} (5)} \end{matrix}$

The above approximate expression is meaningful in considering a concentration boundary layer where the concentration change in the diffusion layer starts. This expression is further solved about δ, Mathematical expression (6) is obtained.

$\begin{matrix} \left\lbrack {{Math}\mspace{14mu} 6} \right\rbrack & \; \\ {\delta = \frac{{DC}\; \max}{J}} & {{Mathematical}\mspace{14mu} {expression}\mspace{14mu} (6)} \end{matrix}$

J represents total mass transfer amount. Next, when J in Mathematical expression (6) is regarded as a mass transfer value per unit area and the solution of the above-described one-dimensional diffusion equation (Mathematical expression (3)) is substituted into J, the following Mathematical expression (7) is obtained.

[Math 7]

δ=√{square root over (πDt)}  Mathematical expression (7)

Mathematical expression (7) takes a form of a square root with respect to the reaction time t. The diffusion layer thickness is considered to increase with time.

A chart of the solution obtained by actually substituting the diffusion coefficient of carbon dioxide is illustrated in FIG. 5. It is found that the thickness of the diffusion layer increases with the reaction time. On the assumption of a conversion efficiency of 10%, the surface current density to achieve is about 10 mA/cm². It can be estimated that the reaction layer thickness is about 100 to 200 μm in 1 to 2 sec.

Next, fluidity will be described. The laminar flow is a flow governed by viscous force and, for example, in an experiment using a dyeing agent, the dyeing agent put into a middle portion flows as it is in a straight line in the middle portion. More specifically, assuming that a velocity direction with a traveling direction regarded as positive is an X-direction and a direction perpendicular to the velocity direction is a Y-direction, almost all of velocity components are only in the X-direction. In contrast to this, the turbulent flow is a flow governed by inertial force, and the dyeing agent generates a vortex or turbulence after released into the center, and finally dissolved in the fluid evenly. This is because the turbulent flow has a velocity component in the Y-direction in addition to the velocity component in the X-direction, and turbulence that can be called stirring occurs in the fluid.

Navie-Stokes equation being a primitive equation of fluid is not an equation in which a smooth general solution is obtained at the present stage. The solution can be found only at the limited boundary while limiting to the laminar flow, namely, an extremely slow flow. One solution is the velocity distribution in the laminar flow. For example, the velocity distribution of the laminar flow flowing through a cylindrical tube is expressed by the following expression where a cylindrical tube center is R, an arbitrary point from a wall surface (r=0) is r.

$\begin{matrix} \left\lbrack {{Math}\mspace{14mu} 8} \right\rbrack & \; \\ {U = {{AU}\; {\max \left( {1 - \left( \frac{r}{R} \right)^{2}} \right)}}} & {{Mathematical}\mspace{14mu} {expression}\mspace{14mu} (8)} \end{matrix}$

U represents arbitrary velocity. A represents integral constant that adjusts the whole flow velocity to an average value of U. In the case of a cylindrical tube, A=2. Umax represents velocity at a point of the largest flow velocity at the center in the cylindrical tube. Exponent of U is 2. Accordingly, this curve is a parabola with 0 at a wall surface r=0 and the largest point at a center r=R. More specifically, it is found that the flow at the wall surface of the cylindrical tube is extremely slow because it is governed by viscosity, but becomes gradually faster as it goes to the center on the other hand. Further, in the experiment using the dyeing agent, assuming that the flow velocity direction of the laminar flow is X, it is known that the dyeing agent put into the center flows on one straight line almost at the center and has little or no flow component in the Y-direction perpendicular to the main flow velocity.

The turbulent flow has no solution of Navie-Stokes equation and is often derived based on the empirical rule. An example of the equation showing the velocity distribution of a representative turbulent flow is an equation called one-seventh power law (Mathematical expression (9)).

$\begin{matrix} \left\lbrack {{Math}\mspace{14mu} 9} \right\rbrack & \; \\ {U = {{AU}\; {\max \left( \frac{r}{R} \right)}^{\frac{1}{7}}}} & {{Mathematical}\mspace{14mu} {expression}\mspace{14mu} (9)} \end{matrix}$

U represents velocity at an arbitrary r, and A represents integral constant that adjusts the whole flow velocity to an average value of U. Umax represents velocity at a point of the largest flow velocity at the center in the cylindrical tube. Umax represents velocity at a point of the largest velocity at the center of the cylindrical tube. In the above flow velocity distribution, the time average is in the main flow velocity direction X. In the above-described experiment using the dyeing agent, the dyeing agent at the center of the tube is immediately distributed in the whole region in the tube. In other words, the turbulent flow has, in addition to the velocity component in the X-direction, many velocity components in the Y-direction perpendicular thereto. In this point, in the case of diffusion of carbon dioxide in the above, carbon dioxide is considered to tend to move to the whole in the tube due to turbulence caused by the velocity components in the Y-direction in the turbulent flow, relative to the laminar flow that tends to make a steady state.

Flow is classified, in terms of hydrodynamics, into three flows such as a laminar flow, a turbulent flow, and a transition flow located between them. The index to be used for the classification is dimensionless number Reynolds number (Re). The reason why the dimensionless number Reynolds number can be used as the index is that the sole variable in the Navie-Stokes equation being a primitive equation of fluid is the Reynolds number. The Reynolds number is a dimensionless number and known as an index that indicates the dynamic flow similarity also for a flow different in flow velocity. Reynolds number (Re) is expressed by the following expression (Mathematical expression (10)).

$\begin{matrix} \left\lbrack {{Math}\mspace{14mu} 10} \right\rbrack & \; \\ {{Re} = \frac{dvL}{\mu}} & {{Mathematical}\mspace{14mu} {expression}\mspace{14mu} (10)} \end{matrix}$

Re represents dimensionless number, d represents density of a solution, v represents flow velocity, L represents representative length, and μ represents viscosity of a solution. There are various units for density, flow velocity, and viscosity, but it is necessary to perform calculation by adjusting the order so that the unit necessarily becomes dimensionless in calculation of Reynolds number (so that the respective units of weight, time, length, and mass cancel one another). Though depending on the shape of the flow path, generally, a flow with a Reynolds number of 2300 or less is the laminar flow, a flow with a Reynolds number of 3000 or more is the turbulent flow, and a flow with a Reynolds number of more than 2300 and less than 3000 is the transition flow.

The representative length L is a length representing the flow path for fluid, and is generally decided customarily. For example, in the case of a flow in the cylindrical tube, the representative length L is expressed by the diameter of the cylindrical tube. In the case of parallel plates, the representative length L is expressed by twice the distance between the parallel plates. In the case of a flow in a tube having a rectangular cross-sectional area, a flow path cross-sectional area is mathematically converted into a shape corresponding to a circle, and the representative length L is expressed by the diameter of a corresponding cylindrical tube after the conversion. This embodiment follows these customs.

The velocity distribution of the laminar flow and the velocity distribution of the turbulent flow in the cylindrical tube are expressed as in FIG. 6 when the two flows are compared in difference in velocity distribution with a dimensionless flow velocity obtained by dividing a generalized flow velocity U by an average flow velocity Uaverage plotted on the vertical axis, and the center of the cylindrical tube being R and an arbitrary point from the wall surface (r=0) being r plotted on the horizontal axis. FIG. 6 is a chart illustrating the relationship between the radius of the cylindrical tube and the dimensionless velocity. The laminar flow exhibits a gentle rise from the wall surface of the cylindrical tube. In contrast to this, the turbulent flow exhibits a very sharp rise from the wall surface, namely, the surface of the electrode when applied to the reaction electrode, and is found to have a flow velocity of about 0.5 to 0.6 times the average flow velocity even at a point corresponding to 1/100 from the surface of the electrode to the center of the cylindrical tube.

Considering the above, before carbon dioxide reaches the diffusion rate-controlling, namely, in a diffusion layer thickness of 100 to 200 μm being the value corresponding to 1 to 2 sec in terms of reaction time, when the radius of the cylindrical tube is 1 cm, a region of 50 to 100 μm where the velocity distribution is generated at a position of 1/100 from the wall surface of the flow path in the turbulent flow is a velocity boundary layer. The velocity boundary layer is predicted to exist in the diffusion layer. In other words, supply of fresh carbon dioxide by a flow field due to the turbulent flow prevents a diffusion layer from being formed and expanding with time to avoid the diffusion rate-controlling. This shows that it is necessary to introduce the turbulent flow into the electrolytic solution flow path for sending solution.

To increase the reaction efficiency, it is necessary not only to make the turbulent flow but also to consider the transport rate-controlling. To make the turbulent flow, it is only necessary to simply increase the flow velocity. However, the pump is limited in discharge flow rate, and needs to continuously send a huge amount of electrolytic solution to form the turbulent flow. The surface of the electrode reacts, but most of the main flow does not react but flows to the outside of the device.

To make the turbulent flow, it is only necessary, for example, to decrease the flow path cross-sectional area. Considering use of the pump generating a flow with the same volume flow rate, to change the flow from the laminar flow to the turbulent flow, it is only necessary to decrease the flow path cross-sectional area. As a result, the flow velocity increases to increase Reynolds number, thereby enabling formation of the turbulent flow. For example, the distance from the surface of the electrode to the wall surface is decreased from 10 cm to 1 mm. In this case, by a flow from the pump having the same volume flow rate, Reynolds number increases by two digits, and when Reynolds number is more than 3000, the turbulent flow is made.

However, in the case of a 1 m square device under consideration of the size when considering commercialization such as making plant, assuming that the solubility of dissolved carbon dioxide is 0.034 mol/l at 25° C., the amount of the electrolytic solution existing in the flow path is 1 m×1 m×0.001 m=0.001 m³=11, so that the absolute amount of carbon dioxide existing in the storage part is 0.034 mol. Further, when trying to react at 10 mA/cm², all carbon dioxide thoroughly reacts in one second, easily reaching the transport rate-controlling of substance.

The problem of the transport rate-controlling can be solved by providing a portion for storing the electrolytic solution, namely, a buffering portion in the flow path in addition to the portion where the flow path cross-sectional area is decreased in consideration of the above situation. In the electrochemical reaction device in this embodiment, reaction is intensively caused at the minimum part of the electrolytic solution flow path, and then consumed carbon dioxide is compensated for in the flowing solution at the maximum part being a pool portion. In other words, the maximum part fulfills a role as the storage part for carbon dioxide to serve to compensate for carbon dioxide in the solution rapidly consumed at the turbulent portion.

Generation of the turbulent flow by providing the minimum part and the maximum part of the flow path only needs to be performed on the reduction side. This is because the reactant on the oxidation side is water in the solvent itself, causing no diffusion rate-controlling. Accordingly, the electrolytic solution flowing through the electrolytic solution flow path on the oxidation side may also be, for example, the laminar flow.

In the electrochemical reaction device illustrated in FIG. 1 and FIG. 2, the electrolytic solution flow path 52 preferably has a smallest value of the area distribution from one end to the other end on the length direction of the electrolytic solution flow path 52 of the cross section perpendicular to the length direction larger than the minimum value at the minimum part of the electrolytic solution flow path 51. The smallest value may be the maximum value or more of the electrolytic solution flow path 51. In this case, the flow of the electrolytic solution 22 in the electrolytic solution flow path 52 is, for example, the laminar flow.

If a failure is caused due to a pressure difference between both flow paths, the electrolytic solution on the oxidation side may be pressurized. To generate the turbulent flow in the electrolytic solution flow path on the reduction side, it is necessary to increase the discharge pressure and the discharge amount of the pump. For this end, energy needs to be put, but in this regard, it is unnecessary to put such excessive energy to the electrolytic solution flow path on the oxidation side, and it is only necessary to form an ordinary electrolytic solution flow path and cause the electrolytic solution to flow in the laminar flow.

In the case of generating the turbulent flow in contrast to the laminar flow, the pressure loss is apt to increase. Accordingly, there is preferably a surplus of discharge pressure. When the electrochemical reaction device is installed on a slope or is constructed to directly stand thereon, the acceleration due to gravity can be utilized for assisting the fluid flow.

As another method for generating the turbulent flow, for example, random packing of a filling material, formation of a protruding structure or the like can be considered. Any of these methods covers the reduction electrode or decreases its effective area. Unlike them, the turbulent flow forming method by the minimum part in the electrochemical reaction device in this embodiment is capable of exposing all the reduction electrode as a reaction surface and can minimize the loss due to the exposure.

On the other hand, in the case where the reduction electrode 31 has a porous structure, it is also conceivable that the electrolytic solution 21 flows through the porous inside. In this case, the structure of the porous body can be analyzed to calculate Reynolds number from the total flow path area and the discharge flow rate. The case where Reynolds number satisfies the range of the turbulent flow is included in the range of the electrochemical reaction device in this embodiment. In this case, the condition of the turbulent flow is that the inside of the porous body is minimum. Accordingly, the reduction electrode 31 may be in contact with the inner wall of the storage part 111. Further, if the reaction rate-controlling is not reached only by the minimum part, a case only having the minimum part being a turbulent flow formation part may be included in the range of the electrochemical reaction device in this embodiment.

Reynolds number of the minimum part 51 b of the electrolytic solution flow path 51 is preferably 3000 or more. From the viewpoint of protecting the reduction electrode 31 from physical breakage and protecting the flow path structure from breakage, Reynolds number of the minimum part 51 b is preferably 200000 or less, and more preferably 20000 or less.

To increase Reynolds number, the flow velocity is increased from Mathematical expression (10), the representative length is increased or the like. The flow velocity of the electrolytic solution 21 at the minimum part 51 b is preferably 1 m/s or more and 100 m/s or less. When the flow velocity is less than 1 m/s, the turbulent flow is hardly generated, and the laminar flow or the transitional flow may be generated in place of the turbulent flow. When the flow velocity is more than 100 m/s, the structural stress on the flow path, the partition wall and the like is large, and breakage of the flow path, peeling of the partition wall and the like may occur.

The representative length corresponds, for example, to the diameter in the case of the cylindrical tube, and corresponds to the interval between the surface 311 of the reduction electrode 31 and the inner wall of the storage part 11 when the minimum part is provided. When the minimum part 51 b is provided, the largest diameter of the cross section perpendicular to the length direction at the minimum part 51 b is preferably 1 mm or more in the case of a cylindrical tube of 20 cm, from one hundredth of the radius of the cylindrical tube being a value where the velocity distribution near the wall surface in the turbulent flow is generated as described above. In consideration of the reaction efficiency of carbon dioxide flowing through the electrolytic solution flow path 51 (the ratio of reacting carbon dioxide in carbon dioxide in the flow), the largest diameter of the cross section perpendicular to the length direction at the minimum part 51 is 50 mm or less, and preferably 25 mm or less. If the surface 311 comes into contact with the inner wall of the storage part 111 in the electrolytic solution flow path 51, the kinetic current density decreases. Therefore, the surface 311 may be apart from the inner wall of the storage part 111.

The aspect ratio between the minimum part and the maximum part can be expressed by the ratio of a largest interval D1 from the surface 311 of the reduction side electrode 31 to the inner wall of the storage part 111 at the maximum part 51 a, to a smallest interval D2 from the surface 311 of the reduction side electrode 31 to the inner wall of the storage part 111 at the minimum part 51 b. In this case, the aspect ratio between the minimum part 51 b and the maximum part 51 a is preferably 10 or more and 1000 or less. When the aspect ratio is less than 10, the absolute amount of carbon dioxide pooled at the maximum part 51 a decreases, making it difficult to supply enough carbon dioxide to the whole device. When the aspect ratio is more than 1000, the amount of the solution necessary for the pool part becomes huge, rapidly increasing the amount of the solution necessary for the reaction.

As described above, in the electrochemical reaction device in this embodiment, the maximum part and the minimum part are provided in the electrolytic solution flow path on the reduction side to combine the turbulent flow and the laminar flow, thereby facilitating supply of carbon dioxide to the surface of the reduction electrode. Accordingly, the problem caused by the diffusion rate-controlling can be solved.

Structural examples of the components in the electrochemical reaction device will be further described. As the electrolytic solution containing water applicable to the electrolytic solution, for example, an aqueous solution containing an arbitrary electrolyte can be used. This solution is preferably an aqueous solution accelerating an oxidization reaction of water. Examples of the aqueous solution containing an electrolyte include aqueous solutions containing phosphoric acid ions (PO₄ ²⁻), boric acid ions (BO₃ ³⁻), sodium ions (Na⁺), potassium ions (K⁺), calcium ions (Ca²⁺), lithium ions (Li⁺) cesium ions (Cs⁺), magnesium ions (Mg²⁺), chloride ions (Cl⁻), hydrogen carbonate ions (HCO³⁻) and so on.

Examples of the electrolytic solution containing carbon dioxide applicable to the electrolytic solution include: salts of a strong base and a strong acid or a weak acid such as potassium hydrogen carbonate, sodium hydrogen carbonate, potassium carbonate, potassium phosphate, dipotassium phosphate, tripotassium phosphate, sodium phosphate, disodium phosphate, trisodium phosphate, sodium borate, disodium borate, potassium borate, dipotassium borate, sodium sulfate, potassium sulfate, potassium chlorate, sodium chlorate, potassium nitrate, and sodium nitrate; acids such as sulfuric acid, hydrochloric acid, boric acid, phosphoric acid, carbonic acid, and nitric acid; and strong bases such as sodium hydroxide and potassium hydroxide, and so on. The electrolytic solution containing carbon dioxide may contain alcohols such as methanol, ethanol, and acetone, or ketones. The electrolytic solution containing water may be the same as the electrolytic solution containing carbon dioxide. However, preferably, the absorption amount of carbon dioxide in the electrolytic solution containing carbon dioxide is high. Accordingly, as the electrolytic solution containing carbon dioxide, a solution different from the electrolytic solution containing water may be used. The electrolytic solution containing carbon dioxide is preferably an electrolytic solution that decreases the reduction potential of carbon dioxide, has high ion conductivity, and contains a carbon dioxide absorbent that absorbs carbon dioxide.

As the above-described electrolytic solution, for example, an ionic liquid which is made of a salt of cations such as an imidazolium ion or a pyridinium ion and anions such as BF₄ ⁻ or PF₆ ⁻ and which is in a liquid state in a wide temperature range, or its aqueous solution can be used. Other examples of the electrolytic solution include amine solutions of ethanolamine, imidazole, and pyridine, or aqueous solutions thereof. Examples of amine include primary amine, secondary amine, and tertiary amine. These electrolytic solutions may have high ion conductivity, have a property of absorbing carbon dioxide, and have characteristics of decreasing the reduction energy.

Examples of the primary amine include methylamine, ethylamine, propylamine, butylamine, pentylamine, hexylamine, and the like. Hydrocarbons of the amine may be substituted by alcohol, halogen, or the like. Examples of the amine whose hydrocarbons are substituted include methanolamine, ethanolamine, chloromethyl amine, and so on. Further, an unsaturated bond may exist. These hydrocarbons are the same in the secondary amine and the tertiary amine.

Examples of the secondary amine include dimethylamine, diethylamine, dipropylamine, dibutylamine, dipentylamine, dihexylamine, dimethanolamine, diethanolamine, dipropanolamine, and so on. The substituted hydrocarbons may be different. This also applies to the tertiary amine. Examples in which the hydrocarbons are different include methylethylamine, methylpropylamine, and so on.

Examples of the tertiary amine include trimethylamine, triethylamine, tripropylamine, tributylamine, trihexylamine, trimethanolamine, triethanolamine, tripropanolamine, tributanolamine, tripropanolamine, triexanolamine, methyl diethylamine, methyldipropylamine, and so on.

Examples of the cation of the ionic liquid include a 1-ethyl-3-methylimidazolium ion, a 1-methyl-3-propylimidazolium ion, a 1-butyl-3-methylimidazole ion, a 1-methyl-3-pentylimidazolium ion, a 1-hexyl-3-methylimidazolium ion, and so on.

Note that a second place of the imidazolium ion may be substituted. Examples of the cation having the imidazolium ion in which second place is substituted include a 1-ethyl-2,3-dimethylimidazolium ion, a 1-2-dimethyl-3-propylimidazolium ion, a 1-butyl-2,3-dimethylimidazolium ion, a 1,2-dimethyl-3-pentylimidazolium ion, a 1-hexyl-2,3-dimethylimidazolium ion, and so on.

Examples of the pyridinium ion include methylpyridinium, ethylpyridinium, propylpyridinium, butylpyridinium, pentylpyridinium, hexylpyridinium, and so on. In both of the imidazolium ion and the pyridinium ion, an alkyl group may be substituted, or an unsaturated bond may exist.

Examples of the anion include a fluoride ion, a chloride ion, a bromide ion, an iodide ion, BF₄ ⁻, PF₆ ⁻, CF₃COO⁻, CF₃SO₃ ⁻, NO₃ ⁻, SCN⁻, (CF₃SO₂)₃C⁻, bis(trifluoromethoxysulfonyl)imide, bis(trifluoromethoxysulfonyl)imide, bis(perfluoroethylsulfonyl)imide, and so on. A dipolar ion in which the cation and the anion of the ionic liquid are coupled by hydrocarbons may be used. Note that a buffer solution such as a potassium phosphate solution may be supplied to the storage parts 111, 112.

FIG. 7 is a schematic sectional view illustrating a structural example of a photoelectric conversion cell. The photoelectric conversion cell illustrated in FIG. 7 includes a conductive substrate 30, the reduction electrode 31, the oxidation electrode 32, the photoelectric conversion body 33, a light reflector 34, a metal oxide body 35, and a metal oxide body 36.

The conductive substrate 30 is provided to be in contact with the reduction electrode 31. Note that the conductive substrate 30 may be regarded as a part of the reduction electrode. An example of the conductive substrate 30 is a substrate containing at least one or a plurality of Cu, Al, Ti, Ni, Fe, and Ag. For example, a stainless substrate including a stainless steel such as SUS may be used. The conductive substrate 30 is not limited thereto, and may be constituted using a conductive resin. Besides, the conductive substrate 30 may be constituted using a semiconductor substrate such as Si or Ge. Further, a resin film or the like may be used as the conductive substrate 30. For example, a membrane applicable to the ion exchange membrane 4 may be used as the conductive substrate 30.

The conductive substrate 30 has a function as a supporter. The conductive substrate 30 may be provided so as to separate the storage part 111 and the storage part 112. The provision of the conductive substrate 30 can improve the mechanical strength of the photoelectric conversion cell 3. Besides, the conductive substrate 30 may be regarded as a part of the reduction electrode 31. Further, the conductive substrate 30 does not necessarily have to be provided.

The reduction electrode 31 preferably contains a reduction catalyst. The reduction electrode 31 may contain both a conductive material and the reduction catalyst. Examples of the reduction catalyst include materials decreasing activation energy to reduce the hydrogen ions and carbon dioxide. In other words, the examples include materials which lower overvoltage when hydrogen and carbon compounds are generated by the reduction reaction of the hydrogen ions and carbon dioxide. For example, a metal material or a carbon material can be used. As the metal material, for example, a metal such as platinum, nickel, or an alloy containing the metal can be used in the case of hydrogen. In the reduction reaction of carbon dioxide, a metal such as gold, aluminum, copper, silver, platinum, palladium, or nickel, or an alloy containing the metal can be used. As the carbon material, for example, graphene, carbon nanotube (CNT), fullerene, ketjen black, or the like can be used. Note that the reduction catalyst is not limited thereto, and, for example, a metal complex such as a Ru complex or a Re complex, or an organic molecule having an imidazole skeleton or a pyridine skeleton may be used as the reduction catalyst. Besides, a plurality of materials may be mixed.

The oxidation electrode 32 preferably contains an oxidation catalyst. The oxidation electrode 32 may contain both a conductive material and the oxidation catalyst. Examples of the oxidation catalyst include materials decreasing activation energy to oxidize water. In other words, the examples include materials which lower overvoltage when oxygen and hydrogen ions are generated by the oxidation reaction of water. The examples include iridium, platinum, cobalt, manganese, and the like. Besides, as the oxidation catalyst, a binary metal oxide, a ternary metal oxide, a quaternary metal oxide, or the like can be used. Examples of the binary metal oxide include manganese oxide (Mn—O), iridium oxide (Ir—O), nickel oxide (Ni—O), cobalt oxide (Co—O), iron oxide (Fe—O), tin oxide (Sn—O), indium oxide (In—O), ruthenium oxide (Ru—O), and so on. Examples of the ternary metal oxide include Ni—Co—O, La—Co—O, Ni—La—O, Ni—Fe—O, Sr—Fe—O, and so on. Examples of the quaternary metal oxide include Pb—Ru—Ir—O, La—Sr—Co—O, and so on. Note that the oxidation catalyst is not limited thereto, and a metal complex such as a Ru complex or a Fe complex can also be used as the oxidation catalyst. Besides, a plurality of materials may be mixed.

At least one of the reduction electrode 31 and the oxidation electrode 32 may have a porous structure. Examples of the material applicable to the electrode having the porous structure include a carbon black such as ketjen black and VULCAN XC-72, activated carbon, metal fine powder, and so on in addition to the above-described materials. The area of an activation surface which contributes to the oxidation-reduction reaction can be made large by having the porous structure, and therefore, the conversion efficiency can be increased.

When an electrode reaction with low current density is performed by using relatively low light irradiation energy, there are many options in catalyst material. Accordingly, for example, it is easy to perform a reaction by using a ubiquitous metal or the like, and it is also relatively easy to obtain selectivity of the reaction. On the other hand, when the photoelectric conversion body 33 is not provided in the electrolytic solution tank 11, but the photoelectric conversion body 33 is electrically connected to at least one of the reduction electrode 31 and the oxidation electrode 32 by a wire or the like, an electrode area generally becomes small for a reason of miniaturizing the electrolytic solution tank or the like, and the reaction is performed with high current density in some cases. In this case, it is preferable to use a noble metal as the catalyst.

The photoelectric conversion body 33 has a stacked structure including a photoelectric conversion layer 33 x, a photoelectric conversion layer 33 y, and a photoelectric conversion layer 33 z. The number of stacked photoelectric conversion layers is not limited to that illustrated in FIG. 7.

The photoelectric conversion layer 33 x includes, for example, an n-type semiconductor layer 331 n containing n-type amorphous silicon, an i-type semiconductor layer 331 i containing intrinsic amorphous silicon germanium, and a p-type semiconductor layer 331 p containing p-type microcrystal silicon. The i-type semiconductor layer 331 i is a layer which absorbs light in a short wavelength region including, for example, 400 nm. Accordingly, charge separation occurs at the photoelectric conversion layer 33 x due to the light energy in the short wavelength region.

The photoelectric conversion layer 33 y includes, for example, an n-type semiconductor layer 332 n containing n-type amorphous silicon, an i-type semiconductor layer 332 i containing intrinsic amorphous silicon germanium, and a p-type semiconductor layer 332 p containing p-type microcrystal silicon. The i-type semiconductor layer 332 i is, for example, a layer which absorbs light in an intermediate wavelength region including 600 nm. Accordingly, the charge separation occurs at the photoelectric conversion layer 33 y due to the light energy in the intermediate wavelength region.

The photoelectric conversion layer 33 z includes, for example, an n-type semiconductor layer 333 n containing n-type amorphous silicon, an i-type semiconductor layer 333 i containing intrinsic amorphous silicon, and a p-type semiconductor layer 333 p containing p-type microcrystal silicon. The i-type semiconductor layer 333 i is, for example, a layer which absorbs light in a long wavelength region including 700 nm. Accordingly, the charge separation occurs at the photoelectric conversion layer 33 z due to the light energy in the long wavelength region.

The p-type semiconductor layer or the n-type semiconductor layer can be formed by, for example, adding an element to be donor or acceptor into the semiconductor material. Note that the semiconductor layer containing silicon, germanium, or the like is used as the semiconductor layer in the photoelectric conversion layer, but is not limited thereto, and for example, a compound semiconductor layer or the like can be used. As the compound semiconductor layer, for example, a semiconductor layer containing GaAs, GaInP, AlGaInP, CdTe, CuInGaSe, or the like can be used. Besides, a layer containing a material such as TiO₂ or WO₃ may be used as long as it can perform the photoelectric conversion. Further, each semiconductor layer may be single crystalline, polycrystalline, or amorphous. Besides, a zinc oxide layer may be provided in the photoelectric conversion layer.

The light reflector 34 is provided between the conductive substrate 30 and the photoelectric conversion body 33. An example of the light reflector 34 is a distribution Bragg reflector composed of, for example, a stack of metal layers or semiconductor layers. The provision of the light reflector 34 makes it possible to reflect the light which could not be absorbed by the photoelectric conversion body 33, and cause the light to enter any of the photoelectric conversion layer 33 x to the photoelectric conversion layer 33 z, thereby increasing the conversion efficiency from light to chemical substances. As the light reflector 34, for example, a layer of a metal such as Ag, Au, Al, Cu, an alloy containing at least one of these metals, or the like can be used.

The metal oxide body 35 is provided between the light reflector 34 and the photoelectric conversion body 33. The metal oxide body 35 has a function of, for example, adjusting an optical distance to increase the light reflectivity. As the metal oxide body 35, it is preferable to use a material which can come into ohmic-contact with the n-type semiconductor layer 331 n. As the metal oxide body 35, for example, a layer of light-transmissive metal oxide such as an indium tin oxide (ITO), zinc oxide (ZnO), fluorine-doped tin oxide (FTO), aluminum-doped zinc oxide (AZO), or antimony-doped tin oxide (ATO) can be used.

The metal oxide body 36 is provided between the oxidation electrode 32 and the photoelectric conversion body 33. The metal oxide body 36 may be provided at a surface of the photoelectric conversion body 33. The metal oxide body 36 has a function as a protective layer which suppresses breakage of the photoelectric conversion cell 3 due to the oxidation reaction. The provision of the metal oxide body 36 makes it possible to suppress corrosion of the photoelectric conversion body 33, and elongate an operating life of the photoelectric conversion cell 3. Note that the metal oxide body 36 does not necessarily have to be provided.

As the metal oxide body 36, for example, a dielectric thin film such as TiO₂, ZrO₂, Al₂O₃, SiO₂, or HfO₂ can be used The thickness of the metal oxide body 36 is 10 nm or less, and 5 nm or less. To cause the tunnel effect, the thickness of the metal oxide body 36 is preferably 2 nm or less, and more preferably 1 nm or less. As the metal oxide body 36, for example, a layer of a light transmissive metal oxide such as an indium tin oxide (ITO), zinc oxide (ZnO), fluorine-doped tin oxide (FTO), aluminum-doped zinc oxide (AZO), or antimony-doped tin oxide (ATO) may be used.

In the case of a TiO₂ film produced at about 150° C. by the atomic layer deposition (ALD) method, a film achieving both transparency and conductive property can be obtained by shortening the precursor exhaust time by the pump. This case may be considered to be normal electron carrier conduction since not the tunneling conduction but carbon atoms remaining in the TiO₂ film are considered to act as dopants. The film thickness in this case may be several tens of nanometers or more and several hundreds of nanometers or less. However, since multilayering or increasing the film thickness causes high wavelength dependency of light, the film thickness needs to be optically carefully calculated. Specifically, when the wavelength is A and the refractive index is n, it is preferable to form a λ/4 anti-reflection film structure composed of m times λ/4n, m being an odd number such as 1, 3, 5.

The metal oxide body 36 may have, for example, a structure where a metal and a transparent conductive oxide are stacked, a structure where a metal and another conductive material are complexed, or a structure where a transparent conductive oxide and another conductive material are complexed. The above structure makes it possible to reduce the number of parts and weight, make it easy to manufacture, and reduce the cost. The metal oxide body 36 may have functions as a protective layer, a conductive layer, and a catalyst layer.

In the photoelectric conversion cell 3 illustrated in FIG. 7, a surface of the n-type semiconductor layer 331 n opposite to a contact surface with the i-type semiconductor layer 331 i is the surface 331 of the photoelectric conversion body 33, and a surface of the p-type semiconductor layer 333 p opposite to a contact surface with the i-type semiconductor layer 333 i is the surface 332. As described above, by stacking the photoelectric conversion layer 33 x to the photoelectric conversion layer 33 z, the photoelectric conversion cell 3 illustrated in FIG. 7 can absorb the light in a wide wavelength range of the sunlight and more effectively utilize the solar energy. At this time, respective photoelectric conversion layers are connected in series, and therefore high voltage can be obtained.

In FIG. 7, the electrodes are stacked on the photoelectric conversion body 33, and therefore the charge-separated electrons and holes can be utilized as they are for the oxidation-reduction reaction. Besides, it is unnecessary to electrically connect the photoelectric conversion body 33 and the electrodes by the wire or the like. It is therefore possible to perform the oxidation-reduction reaction with high efficiency.

A plurality of photoelectric conversion bodies may be electrically connected in parallel connection. A two-junction type, single-layer type photoelectric conversion bodies may be used. A stack of two or four or more photoelectric conversion bodies may be provided. A single photoelectric conversion layer may be used instead of the stack of the plurality of photoelectric conversion layers.

The electrochemical reaction device in this embodiment is a simplified system, in which the reduction electrode, the oxidation electrode, and the photoelectric conversion body are integrated to reduce the number of parts. Accordingly, for example, at least any one of manufacture, installation, and maintenance becomes easy. Further, the wires or the like connecting the photoelectric conversion body with the reduction electrode and the oxidation electrode become unnecessary, and therefore it is possible to increase the light transmittance, and enlarge the light receiving area.

There is a case where the photoelectric conversion body 33 is corroded because it is in contact with the electrolytic solution, and a corrosion product is dissolved in the electrolytic solution to cause deterioration of the electrolytic solution. To prevent the corrosion, provision of a protective layer can be considered. However, there is a case where a protective layer component is dissolved in the electrolytic solution. Hence, a filter such as a metal ion filter is provided in the flow path or the electrolytic solution tank to suppress the deterioration of the electrolytic solution.

The shape of the partition wall 6 is not limited to that illustrated in FIG. 1. An electrochemical reaction device illustrated in FIG. 8 includes a partition wall 6 having a maximum part 51 a that has a rectangular shape in the cross section in the length direction. An electrochemical reaction device illustrated in FIG. 9 includes a partition wall 6 having a maximum part 51 a that has an inverted triangular shape in the cross section in the length direction. An electrochemical reaction device illustrated in FIG. 10 includes a partition wall 6 having a maximum part 51 a that has a semicircular shape protruding downward in the cross section in the length direction.

The structural example of the electrochemical reaction device is not limited to that in FIG. 1. FIG. 11 is a schematic view illustrating another example of the electrochemical reaction device. The electrochemical reaction device illustrated in FIG. 11 is different, as compared to the electrochemical reaction device illustrated in FIG. 1, at least in a structure that the reduction electrode 31 is not in contact with the surface 331 of the photoelectric conversion body 33. The reduction electrode 31 is electrically connected to the surface 331 via a wire 81. In this case, a part of the wire 81 may be extended to the outside of the electrolytic solution tank 11.

FIG. 12 is a schematic view illustrating another example of the electrochemical reaction device. The electrochemical reaction device illustrated in FIG. 12 is different, as compared to the electrochemical reaction device illustrated in FIG. 1, at least in a structure that the photoelectric conversion body 33 is provided in contact with the outside wall of the electrolytic solution tank 11, and the reduction electrode 31 and the oxidation electrode 32 are not in contact with the photoelectric conversion body 33. The reduction electrode 31 is electrically connected to the surface 331 via a wire 81. The reduction electrode 32 is electrically connected to the surface 332 via a wire 82. In this case, a part of the wire 81 and a part of the wire 82 may be extended to the outside of the electrolytic solution tank 11. The case where the photoelectric conversion body is connected to the reduction electrode or the oxidation electrode via the wire or the like is advantageous in terms of a system because the components are separated for each function.

FIG. 13 is a schematic view illustrating another example of the electrochemical reaction device. The electrochemical reaction device illustrated in FIG. 13 is different, as compared to the electrochemical reaction device illustrated in FIG. 12, at least in a structure including, as the photoelectric conversion body 33, a three-junction type stack of photoelectric conversion layers 33 x, 33 y, 33 z provided outside the storage part 11. The descriptions of the photoelectric conversion layers 33 x, 33 y, 33 z illustrated in FIG. 7 can be appropriately quoted to the descriptions of the photoelectric conversion layers 33 x, 33 y, 33 z. Note that the photoelectric conversion cell may be configured to have the structure illustrated in FIG. 7. In FIG. 13, the surface of a photoelectric conversion layer 33 x on the reduction electrode 31 side is the surface 331, and a surface of a photoelectric conversion layer 33 z on the oxidation electrode 32 side is the surface 332. The reduction electrode 31 is electrically connected to the surface 331 via a wire 81 and a conductive layer 83 provided on the surface 331. The reduction electrode 32 is electrically connected to the surface 332 via a wire 82 and a conductive layer 84 provided on the surface 332. Note that a unit including the conductive layer 83, the photoelectric conversion layers 33 x, 33 y, 33 z, and the conductive layer 84 is also referred to as a photoelectric conversion unit 3. In this case, a part of the wire 81 and a part of the wire 82 may be extended to the outside of the electrolytic solution tank 11. Note that the number of the stacked photoelectric conversion layers may be 2 or 4 or more.

FIG. 14 is a schematic view illustrating another example of the electrochemical reaction device. The electrochemical reaction device illustrated in FIG. 14 is different, as compared to the electrochemical reaction device illustrated in FIG. 13, at least in a structure including a plurality of photoelectric conversion units 3 each including a single junction type photoelectric conversion body 33. The description of the above photoelectric conversion body 33 can be appropriately quoted to the description of the photoelectric conversion body 33. The description of the photoelectric conversion unit 3 can be appropriately quoted to the description of the other structure of the photoelectric conversion unit 3. The plurality of photoelectric conversion units 3 are electrically connected to one another by serial connection. Note that the number of the serially connected photoelectric conversion units 3 may be 2 or 4 or more. By connecting the plurality of photoelectric conversion units 3 in series, the voltage to be used for the oxidation-reduction reaction can be increased.

The above embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. The above embodiments may be embodied in a variety of other forms, and various omissions, substitutions and changes may be made without departing from the spirit of the inventions. The above embodiments and modifications thereof are included in the scope and spirit of the inventions and included in the inventions described in the claims and their equivalents.

EXAMPLE Example 1

An electrochemical reaction device including a structure was fabricated. The structure includes a three-junction type photoelectric conversion body with a thickness of 500 nm, a ZnO layer with a thickness of 300 nm provided on a first surface of the three junction type photoelectric conversion body, an Al layer with a thickness of 200 nm provided on the ZnO layer, a SUS substrate with a thickness of 1.5 mm provided on the Al layer, and an ITO layer with a thickness of 70 nm provided on a second surface of the three-junction type photoelectric conversion body. Each layer of the structure has a texture structure of a submicron order for the purpose of obtaining the light confinement effect.

The three-junction type photoelectric conversion body includes a first photoelectric conversion layer which absorbs light in the short wavelength region, a second photoelectric conversion layer which absorbs light in the intermediate wavelength region, and a third photoelectric conversion layer which absorbs light in the long wavelength region as in the embodiment. The first photoelectric conversion layer includes a p-type microcrystalline silicon layer, an i-type amorphous silicon layer, and an n-type amorphous silicon layer. The second photoelectric conversion layer includes a p-type microcrystalline silicon layer, an i-type amorphous silicon germanium layer, and an n-type amorphous silicon layer. The third photoelectric conversion layer includes a p-type microcrystalline silicon germanium layer, an i-type amorphous silicon layer, and an n-type amorphous silicon layer.

An open-circuit voltage when the structure was irradiated with light using a solar simulator (AM1.5, 1000 W/m²) was measured. The open-circuit voltage was 2.4 V.

An atomic layer deposition apparatus was used to send a pulse alternately with water using an organic Ni compound as a precursor onto the ITO layer with a thickness of 100 nm to perform repeated deaeration to thereby form a nickel oxide layer with a thickness of about 6 nm at 150° C. A copper wire was bonded as a contact layer by a copper tape on the rear surface of the SUS substrate. An Au plate with a thickness of 1 mm was used as the reduction electrode and a potentiogalvanostat was used to apply a high frequency of 10 kHz between the potential where the oxidation of a first wave of two waves ended and the potential where the reduction thereof occurred, in a 1M sulfuric acid. After application for 3 hours, the resultant structure was taken out of the solution and reduced in 0.5M potassium carbonate, whereby a catalyst layer was obtained. In confirmation under SEM (Scanning Electron Microscope), the Au surface had a shape looked like having been etched into a shape of particle of several tens of nanometers. Then, the resultant structure was cut-out into a 1 cm square shape, and an edge part was sealed with a chemical resistant polymer sheet.

A 10 cm square acrylic flow path cell was prepared as the electrolytic solution flow path on the oxidation side, an oxidation electrode having a photoelectric conversion body provided with a catalyst was arranged and fixed in the flow path, and an Au plate being the reduction electrode was arranged and fixed thereto via an anion exchange membrane (Selemion). The oxidation electrode and the reduction electrode were electrically connected via a copper wire, and connected to the potentiogalvanostat. To the oxidation side flow path, a 0.5M potassium carbonate solution deaerated by argon bubbling was sent at a flow rate of 500 sccm (ml/min) by a plunger pump.

A 10 cm square acrylic flow path cell was used as the electrolytic solution flow path on the reduction side. The acrylic flow path cell was designed to be adjustable by an O-ring so that the distance to the inner wall of the flow path cell was 1 mm. Bubbling of carbon dioxide was performed at a flow rate of 200 sccm in a bubbling tower with a volume of 51, and a 0.5M potassium carbonate solution saturated with carbon dioxide was sent into the flow path with a length of 10 cm by the plunger pump at a flow rate of 500 sccm. Reynolds number in the electrolytic solution flow path when the representative length was set to 10 cm was Re 8333. From this, it is considered that the turbulent flow occurs in the electrolytic solution flow path.

The photoelectric conversion body was irradiated with light from the solar simulator (AM1.5, 1000 W/m²). Bubbles were generated from each of the oxidation electrode and the reduction electrode, and gas chromatography was used for the gas generated from the reduction electrode. As a result, the generated gas was carbon monoxide and hydrogen. Further, Faraday's efficiency using carbon monoxide as a reference was about 91%. A reaction potential was obtained from the obtained current value and current density and the reaction amount between hydrogen and carbon monoxide, using the relational expression of Faraday constant (96500 C/mol) and the current density expression (current density=reaction electron number (carbon monoxide and hydrogen are 2)×Faraday constant×potential). In consideration of the intersection with the current-voltage curve of the photoelectric conversion body, the total conversion efficiency from the light irradiation intensity was 3.2%.

Comparative Example

Sending of solution was carried out with the distance between the inner wall of the electrolytic solution flow path on the reduction side and the reduction electrode set to 10 cm in the electrochemical reaction device in Example 1. Reynolds number in the electrolytic solution flow path when the representative length was set to 10 cm was Re 83.

The photoelectric conversion body was irradiated with light from the solar simulator (AM1.5, 1000 W/m²). Bubbles were generated from each of the oxidation electrode and the reduction electrode, and gas chromatography was used for the gas generated from the reduction electrode. As a result, the generated gas was carbon monoxide and hydrogen. Further, Faraday's efficiency using carbon monoxide as a reference was about 80%. A reaction potential was obtained from the obtained current value and current density and the reaction amount between hydrogen and carbon monoxide, using the relational expression of Faraday constant and the current density expression. The total conversion efficiency from the light irradiation intensity was 0.21%. This shows that generation of the turbulent flow in the electrolytic solution flow path can increase the conversion efficiency.

Example 2

A composite substrate (1.5×9 cm square) having a SUS substrate with a thickness of 1.5 mm which was connected to a generator through a conducting wire and a platinum film with a thickness of 100 nm on the SUS substrate, and gold foil (1.5 cm×10 cm square) were prepared. The generator was a simulation device of a solar battery. An electrolytic solution flow path was formed on each of the oxidation electrode side and the reduction electrode side of an acrylic frame of 3 cm×11 cm square with a thickness of 1 cm (inner diameter of 2×10 cm). The composite substrate and the gold foil were embedded in the frame, an ion exchange membrane (Nafion 117, 2×10 cm square) was provided between the composite substrate and the gold foil, and a module sandwiched by a silicon rubber sheet and an acrylic plate (3 cm long×10 cm wide×3 mm thick) was produced at both of the outside of the composite substrate and at the outside of the gold foil. Into the module, a potassium carbonate solution subjected to bubbling in a bubbling tower with argon and then to deaeration was supplied at a flow rate of 500 sccm. The composite substrate was set as the reduction electrode, and the gold foil was set as the oxidation electrode. A part of the acrylic plate was cut by 1 cm×5 cm at three places at an interval of 3 cm using a carving machine, and formed recessed parts are set as maximum parts (pool portions). A resultant structure was installed such that the distance between the inner wall of the flow path cell and the reduction electrode was 1 mm using an O-ring at the surface other than the pool portions. A voltage of 3.6 V was applied between the oxidation electrode and the reduction electrode.

Generation of carbon monoxide was confirmed on the reduction electrode side, showing reduction of carbon dioxide. The exhibited current value was 8.3 mA/cm².

Comparative Example 2

An electrochemical reaction device different, as compared with the electrochemical reaction device in Example 2, in a structure having no pool portion was fabricated. A voltage of 3.6 V was applied between the oxidation electrode and the reduction electrode. Generation of carbon monoxide was confirmed on the reduction electrode side, showing reduction of carbon dioxide. The exhibited current value was 1.6 mA/cm². This shows that the provision of the maximum part as in Example 2 increases the conversion efficiency.

Example 3

An electrochemical reaction device was fabricated by installing photoelectric conversion cells electrically connected by serial connection outside the electrolytic solution tank and installing the reduction electrode and the oxidation electrode inside the electrolytic solution tank. As the photoelectric conversion cell, a photoelectric conversion cell with a 4 cm square in total including a conducting wire part was used, which was made by cutting a single-crystal Si-type solar battery into a 4 cm×7 mm square in a manner to prevent a power transmission line from coming into contact with the outside, connecting four solar batteries after cutting in series by a conducting wire, and then sealing them with glass. Note that the structure of the flow path and the electrolytic solution are the same as those in Example 1.

An open-circuit voltage when the photoelectric conversion body was irradiated with light using a solar simulator (AM1.5, 1000 W/m²) was measured. The open-circuit voltage was 3.6 V, and the photoelectric conversion efficiency was about 13%.

The positive side and negative side conducting layers of the photoelectric conversion body were electrically connected to the oxidation electrode and the reduction electrode respectively. The oxidation electrode and the reduction electrode, in a form of close contact with each other with an anion exchange membrane intervening therebetween, were installed in the electrolytic solution flow path.

A 10 cm square acrylic flow path cell was prepared as the electrolytic solution flow path on the oxidation side, an oxidation electrode having a photoelectric conversion body provided with a catalyst was arranged and fixed in the flow path, and an Au plate being the reduction electrode was arranged and fixed thereto via an anion exchange membrane (Selemion). The oxidation electrode and the reduction electrode were electrically connected via a copper wire, and connected to the potentiogalvanostat. To the oxidation side flow path, a 0.5M potassium carbonate solution deaerated by argon bubbling was sent at a flow rate of 500 sccm (ml/min) by a plunger pump.

A 10 cm square acrylic flow path cell was used as the electrolytic solution flow path on the reduction side. The acrylic flow path cell was designed to be adjustable by an O-ring so that the distance to the inner wall of the flow path cell was 1 mm. Bubbling of carbon dioxide was performed at a flow rate of 200 sccm in a bubbling tower with a volume of 51, and a solution saturated with carbon dioxide was sent into the flow path with a length of 10 cm by the plunger pump at a flow rate of 500 sccm. Reynolds number in the electrolytic solution flow path when the representative length was set to 10 cm was Re 8333. From this, it is considered that the turbulent flow occurs in the electrolytic solution flow path.

The photoelectric conversion body was irradiated with light from the solar simulator (AM1.5, 1000 W/m²). Bubbles were generated from each of the oxidation electrode and the reduction electrode, and gas chromatography was used for the gas generated from the reduction electrode. As a result, the generated gas was carbon monoxide and hydrogen. Further, Faraday's efficiency using carbon monoxide as a reference was about 91%. A reaction potential was obtained from the obtained current value and current density and the reaction amount between hydrogen and carbon monoxide, using the relational expression of Faraday constant and the current density expression. In consideration of the intersection with the current-voltage curve of the photoelectric conversion body, the total conversion efficiency from the light irradiation intensity was 4.7%. This shows that electrical connection of the photoelectric conversion cells in series can increase the conversion efficiency. 

What is claimed is:
 1. An electrochemical reaction device, comprising: an electrolytic solution tank including a first storage part to store a first electrolytic solution containing carbon dioxide, and a second storage part to store a second electrolytic solution containing water; a reduction electrode disposed in the first storage part and having a first surface containing a reduction catalyst; an oxidation electrode disposed in the second storage part and having a second surface containing an oxidation catalyst; and a generator connected to the reduction electrode and the oxidation electrode, wherein a region in the first storage part between the first surface and an inner wall of the first storage part is an electrolytic solution flow path for sending the first electrolytic solution, wherein the electrolytic solution flow path has a maximum part having a maximum value of an area distribution from one end to another end on a length direction of the electrolytic solution flow path of a cross section perpendicular to the length direction, and a minimum part having a minimum value of the area distribution.
 2. The device of claim 1, wherein the generator has a photoelectric conversion body having a third surface connected to the reduction electrode, and a fourth surface connected to the oxidation electrode.
 3. The device of claim 1, wherein Reynolds number of the minimum part is 3000 or more and 200000 or less.
 4. The device of claim 1, wherein an aspect ratio of the maximum part to an aspect ratio of the minimum part is 10 or more and 1000 or less.
 5. The device of claim 1, wherein a flow velocity of the first electrolytic solution at the minimum part when the first electrolytic solution flows through the minimum part is 1 m/s or more and 100 m/s or less.
 6. The device of claim 1, wherein a largest diameter of a cross section perpendicular to the length direction at the minimum part is 1 mm or more and 50 mm or less.
 7. The device of claim 1, further comprising: a partition wall provided in the first storage part between the first surface and the inner wall of the first storage part to be in contact with the inner wall of the first storage part, and is overlaid with the minimum part.
 8. The device of claim 2, wherein the photoelectric conversion body is provided inside the electrolytic solution tank or outside the electrolytic solution tank.
 9. The device of claim 1, wherein a region in the second storage part between the second surface and an inner wall of the second storage part is a second electrolytic solution flow path for sending at least a part of the second electrolytic solution, wherein in the second electrolytic solution flow path, a smallest value of an area distribution from one end to another end on a second length direction of the second electrolytic solution flow path of a cross section perpendicular to the second length direction is larger than the minimum value, and wherein a flow of the second electrolytic solution in the second electrolytic solution flow path is a laminar flow.
 10. The device of claim 1, wherein a compound produced by a reduction reaction of the carbon dioxide contains carbon monoxide.
 11. The device of claim 1, wherein a compound produced by a reduction reaction of the carbon dioxide contains ethylene glycol.
 12. An electrochemical reaction device, comprising: an electrolytic solution tank including a first storage part to store a first electrolytic solution containing carbon dioxide, and a second storage part to store a second electrolytic solution containing water; a reduction electrode disposed in the first storage part and having a first surface containing a reduction catalyst; an oxidation electrode disposed in the second storage part having a second surface containing an oxidation catalyst; and a generator connected to the reduction electrode and the oxidation electrode, wherein a region in the first storage part between the first surface and an inner wall of the first storage part is an electrolytic solution flow path for sending the first electrolytic solution, wherein the electrolytic solution flow path has a first region and a second region and the first and second regions are arranged along a length direction of the electrolytic solution flow path, wherein the second region has a partition wall to cause a turbulent flow in the first electrolytic solution and the partition wall is in contact with the inner wall.
 13. The device of claim 12, wherein Reynolds number of the second region is 3000 or more and 200000 or less.
 14. The device of claim 12, wherein the generator has a photoelectric conversion body having a third surface connected to the reduction electrode, and a fourth surface connected to the oxidation electrode.
 15. The device of claim 14, wherein the photoelectric conversion body is provided inside the electrolytic solution tank or outside the electrolytic solution tank.
 16. The device of claim 12, wherein a compound produced by a reduction reaction of the carbon dioxide contains carbon monoxide.
 17. The device of claim 12, wherein a compound produced by a reduction reaction of the carbon dioxide contains ethylene glycol. 